#include <stdio.h>

int
main ()
{
	int parity = 0;
	int current;

	freopen("deleted.in", "r", stdin);
	freopen("deleted.out", "w", stdout);

	while (scanf("%d", &current) == 1) {
		parity = parity ^ current;
	}
	printf("%d\n", parity);

	return 0;
}
